For the levelling or flattening of the metal strip, various straightening processes are Used, e.g. stretch or tension straightening, tension levelling, stretcher levelling and stretch-bend levelling.
Generally, these straightening or levelling processes are carried out continuously and, although the levelling process has been greatly improved in recent years, even today, it is necessary to tolerate slight central dishing of the strip or edge corrugation following the straightening, flattening or levelling process. The residual lack of planarity results from the nonuniform transverse stresses distributed over the width of the strip and which are superimposed on the tensile stresses in the longitudinal direction of the strip. The smaller the strip thickness and the modulus of elasticity of the strip, the greater is the risk of residual lack of planarity. The same applies to cold rolling of metal strip, especially for the after-rolling or dressing or final rolling stages. Because of the bending of the rolls, as a result of the rolling force, and because the incoming thickness profile of the metal strip is as a rule not truly rectangular, nonuniform deformations can arise in the rolling gap or nip of the rolling mill over the width of the strip, leading to corrugation or waviness in the strip following cold rolling.
As a result, it has been the practice to try to correct the roll gap geometry to eliminate the corrugation or waviness. These efforts, however, have proven to be unsuccessful or insufficient. It appears that the deformation that occurs in the rolling gap does not depend exclusively on the rolling force distribution but is also a function of the tensile stress distribution over the width of the strip. That distribution could not be readily modified heretofore.